Thermal tolerance and the respiratory properties of isolated red muscle mitochondria were investigated in Oreochromis alcalicus grahami from the alkaline hot-springs, Lake Magadi, Kenya. Populations of O. a. grahami were resident in pools at 42.8 &deg;C and migrated into water reaching temperatures of 44.8 &deg;C for short periods. The maximum respiration rates of mitochondria with pyruvate as substrate were 217 and 284 natom O mg-1 mitochondrial protein min-1 at 37 &deg;C and 42 &deg;C, respectively (Q10=1.71). Fatty acyl carnitines (chain lengths C8, C12 and C16), malate and glutamate were oxidised at 70&shy;80 % of the rate for pyruvate. In order to assess evolutionary temperature adaptation of maximum mitochondrial oxidative capacities, the rates of pyruvate and palmitoyl carnitine utilisation in red muscle mitochondria were measured from species living at other temperatures: Notothenia coriiceps from Antarctica (-1.5 to +1 &deg;C); summer-caught Myoxocephalus scorpius from the North Sea (10&shy;15 &deg;C); and Oreochromis andersoni from African lakes and rivers (22&shy;30 &deg;C). State 3 respiration rates had Q10 values in the range 1.8&shy;2.7. At the lower lethal temperature of O. andersoni (12.5 &deg;C), isolated mitochondria utilised pyruvate at a similar rate to mitochondria from N. coriiceps at 2.5 &deg;C (30 natom O mg-1 mitochondrial protein min-1). Rates of pyruvate oxidation by mitochondria from M. scorpius and N. coriiceps were similar and were higher at a given temperature than for O. andersoni. At their normal body temperature (-1.2 &deg;C), mitochondria from the Antarctic fish oxidised pyruvate at 5.5 % and palmitoyl-dl-carnitine at 8.8 % of the rates of mitochondria from the hot-spring species at 42 &deg;C. The results indicate only modest evolutionary adjustments in the maximal rates of mitochondrial respiration in fish living at different temperatures.